1. Field of the Invention
The present invention relates to a data receiver, and more particularly to an improvement of the technique of reproducing a digital signal from received data.
2. Description of the Related Art
In the field of an image forming apparatus, for example, cases where an optical transmission system using an optical fiber cable is employed as a system for transmitting image data of an original which is read by a scanner unit, to a printer unit in a location separated from the scanner unit are increasing.
When image data is to be transmitted by an optical transmission system, a transmitter which converts the image data in the form of a digital signal into an optical signal that is a blinking optical signal, and which then outputs the optical signal to an optical fiber cable is disposed in a scanner unit serving as a transmitting side for the image data, and a receiver which receives the optical signal to reproduce the digital signal is disposed in a printer unit serving as a receiving side.
In the receiver, when the voltage of an input signal which has been photoelectrically converted is not lower than a voltage set as a threshold (hereinafter, such a voltage is referred to as threshold voltage), the signal is judged to be “1,” and, when the voltage is lower than the threshold voltage, the signal is judged to be “0.” In recent years, such a threshold voltage is often set by using an Automatic Threshold Control circuit (hereinafter, referred to as ATC circuit) which automatically adjusts the threshold voltage in accordance with the voltage of an input signal.
An ATC circuit holds the voltage of an input signal by charging an incorporated capacitor, and then outputs a voltage which is about one half of the held voltage, as a threshold signal.
FIG. 1 is a view diagrammatically showing voltage waveforms of an input signal 910 and a threshold signal 920 in a receiver having an ATC circuit.
As shown in the figure, when a signal indicative of “1” is input for a fixed time period as the input signal 910, the voltage of the threshold signal 920 is about one half of that of the signal. When the signal of “1” is followed by an input of a signal indicative of “0,” the voltage of the signal is substantially 0 volt, and hence the voltage of the threshold signal 920 is gradually lowered by discharging of the capacitor. When a signal indicative of “1” is thereafter input, the capacitor is again charged, with the result that the voltage waveform shown in the figure is obtained and an output signal 930 is reproduced. The charging and discharging rates of the capacitor depend on the time constant which is determined by the capacitance of the capacitor and the internal resistance. Usually, the charging and discharging rates are set so that, as shown in the figure, charging and discharging of the capacitor are adequately performed in accordance with the input signal 910 and the threshold voltage is maintained within an appropriate range.
However, image data includes data in which a signal of “0” or “1” is continued for a long time period corresponding to a portion where no image is formed or a solid color portion of an image. When a signal of “0” is received for a long time period, for example, only discharging of the capacitor is performed for a long time period and the threshold voltage is lowered to a substantially 0 volt as indicated by a waveform 921 of the figure. When a signal 911 of “1” is thereafter input, charging of the capacitor is started. In this case, the rising of the threshold voltage requires a time period longer than that in a usual case. Therefore, the signal reproduction is performed before the threshold voltage has not yet been raised to a sufficient level, thereby producing a situation in which the signal of “1” is reproduced earlier by a time period 932 than a signal such as the signal 931 which is to be originally reproduced (hereinafter, the time period 932 is referred to as error time period). In order to prevent such a situation from arising, the capacitance of the capacitor may be reduced to increase the charging rate. When the capacitance of the capacitor is reduced, however, also the discharging rate is increased, and hence the threshold voltage is unstably held. Therefore, the reduction of the capacitance of a capacitor is limited.
The thus reproduced signal 931 tends to cause erroneous detection in sampling. This is more conspicuous as the data transmission speed is higher. When the transmission speed is made higher, the width of a 1-bit signal is shorter. Therefore, the ratio of the error time period 932 with respect to the one-pulse width is larger than that with respect to the one-pulse width in a low transmission speed. When the sampling timing is deviated even by a small degree, consequently, a portion corresponding to the error time period 932 is sampled. In other words, a situation in which a portion that is originally indicative of “0” is erroneously judged to be “1” easily occurs. In the conventional art, therefore, the transmission speed cannot but be set low or to a degree at which, even when a signal of “0” is continued, sampling of a reproduced signal is not adversely affected, with the result that the transmission speed cannot be made higher.